Infants with congenital heart lesions require surgical intervention early in life. Deep hypothermia induced initially by surface cooling followed by cardio-pulmonary bypass is currently being extensively used. The surface cooling phase is very slow, while the cardio-pulmonary bypass phase carries an increased risk of complications. The purpose of the proposed study is to evaluate a combined surface and inhalate temperature controlled method for the induction of hypothermia and rewarming. To facilitate the rate of heat exchange through the body surface and respiratory tract, temperature and humidity gradients, and thermal conductivity of different gas mixtures will be considered. To aid in maintaining satisfactory circulatory dynamics, hypercapnia and hypoxia will be evaluated. The non-invasive simplicity of this method suggests its potential applicability to many clinical situations. BIBLIOGRAPHIC REFERENCES: Beran AV, Proctor KG, Sperling DR: Hypothermia and Rewarming Induced by Surface and HE-O2 Inhalate Temperature Control. J. of Appl. Physiol. 39:2, 1975. Beran, AV, Huxtable, RF, and Sperling, DR: Electrochemical Sensor for Continuous Transcutaneous PCO2 Measurement. J. of Appl. Physiol. 41:442, 1976.